It has been proposed to place electrically conductive material in a seat as a sensor for detecting the presence of an occupant in the seat. For example, U.S. patent application Ser. No. 12/150,439, incorporated herein by reference, describes an occupant detector for a vehicle seat assembly that includes an occupant sensing circuit that measures the impedance of an electric field generated by applying an electric signal to the sensor in the seat. The presence of an occupant affects the electric field impedance about the sensor and is measured by the occupant sensing circuit. In seats equipped with a heating element, the electrically conductive material of the heating element may also serve as the sensor for occupant sensing. Such an arrangement might operate by periodically disconnecting a heating current source from the heating element and connecting the occupant detection circuit. Because of the sensitivity of the measurements required by the occupant sensing circuit, it is necessary to isolate the heating current source from the heating element to prevent interference with the occupant sensing circuit. If the heating control circuit has open-switch impedance that combines with and influences the electric field impedance, the accuracy and reliability of occupant detection is reduced.
It has been proposed to use transistors for connecting and disconnecting heating current to and from the heating element. Even when the transistors are off to disconnect the heating current, the transistors have open-switch impedance that remains coupled to the heating element. Thus, the heating element is not isolated, and the occupant sensing circuit measurement is affected. The effect of the transistor open-switch impedance on the occupant sensing circuit may be reduced by adding diodes in series with the transistors. However, this increases circuit complexity and does not entirely eliminate open-switch impedance because of diode leakage or diode junction capacitance. Moreover, mechanical relays for connecting and disconnecting the heating current comprise relay contacts and interconnecting wires that add open-switch impedance to the occupant sensing circuit, as well as adding to the expense, noise and reliability of the circuit.
What is needed is a seat assembly having occupant detection circuit and seat heating circuit using the same heating element for seat heating and occupant sensing, which uses transistors for heater current control and isolates the transistor open-switch impedance from the occupant detection circuit, thereby minimizing the effect upon occupant detection measurement.